Back on the Big Freeway—with Googly Eyes: Subtracting Three-Digit Numbers with Ungrouping!

They say you should never drive when you’re angry because it makes you an unsafe driver. They never said we couldn’t drive with googly eyes….

Let’s do 862 – 479. No problem! First we’ll draw our freeway lanes. Then we start like we always do—with the ones column!

 

In the ones column, since we can’t subtract 2 − 9, we’ll take 1 ten from the tens column. So that means we change the 6 to a 5, and now we have 12 ones—plenty to subtract with. Now we’re ready to subtract the ones column, and we get 12 − 9 = 3. So far, so good? Great!

But what now? I mean, we can’t subtract 5 − 7! We don’t have enough tens! Well, I’ve heard that there are big packages that hold 10 boxes of googly eyes—and we know the boxes have 10 eyes each, so that’s 100 googly eyes total! Yep, we’re going to take 1 hundred from the hundreds column in order to get 10 tens!

This means we need to cross out the 8 from the hundreds column and make it a 7…and then we need to change the 5 to a 15 in the tens column! Crazy, right? But it works!

Next we subtract the tens column: 15 − 7 = 8, and then the hundreds column is 7 − 4 = 3. We’re left with 3 hundreds, 8 tens, and 3 ones—in other words, 383! Done! Well, sort of…

We’ve just shown that 862 − 479 = 383. But it sure seems like we could have made a mistake somewhere in there, doesn’t it? Let’s check our answer by making a turkey sandwich. (See page 134 if you have no idea what I’m talking about.) So let’s try 479 + 383 and hope we get 862!

Great! That means we got the right answer. Done!

Answer: 862 − 479 = 383

 

Your brain is getting stronger, one math problem at a time!